For male zebra finches, two groups of brain cells interact to help them learn songs: Premotor neurons fire whenever a bird hears his dad's song, and inhibitory interneurons help protect previously learned song segments but not those that are still being refined. As a result, the birds can tune out song segments they’ve already mastered. The study, published in Science last week, suggests that age doesn’t matter as much when it comes to learning new songs. Rather, it’s the firing of inhibitory neurons that really locks the learning into memory.
Many species (us included) learn behaviors by observing and imitating our parents and peers, yet the neural mechanisms underlying this process are still largely unknown. Previous work with zebra finches revealed that when juveniles are exposed to songs by their tutors (typically dad), neurons in a brain region called the HVC act in a way that’s thought to be critical to song imitation. Curiously, the premotor neurons in the HVC respond to new songs in not just juvenile finches, but in adults as well.
Using tiny electrodes, NYU’s Michael Long and colleagues tracked brain cell activity in both adults and juveniles as they learned songs from a tutor bird. They found that, while there was little difference between the two age groups, inhibitory interneuron firing is linked to how recently a finch was taught a new song. The team then used a fake tutor to teach the finches just one syllable, and only after this syllable was perfected were the finches introduced to another. This allowed the researchers to distinguish the inhibition patterns of firing neurons between learned and new syllables.
It turns out, inhibitory interneurons spiked coherently during learned syllables, but not for those syllables that haven’t been learned and recognized yet. This wasn’t the case for just young birds. Older finches who could sing the two syllables well showed equivalent inhibition across both. And the precision of interneuron firing is correlated with song accuracy. The influence of the parent or mentor on a young bird’s nerve circuits gradually decreased as songs were learned correctly, and by the time they reached about 100 days old, the birds were able to ignore their tutors completely.
"While we have known for decades that adolescent songbirds only learn their songs if exposed to a tutor," Long says in a statement, "we believe our study is the first to detail changes in nerve networks that make this mastery possible in maturing brains."
Image in the text: A male teaches a courtship song to a younger finch while the team recorded nerve circuits. NYU Langone Medical Center